1. Field Of The Invention
The present invention relates to a color toner which is useful in image forming methods which include a step in which an electrostatic latent image is developed such as electrophotography, electrostatic recording and electrostatic printing, and to a method of manufacture of the color toner and to a method of image formation using the color toner.
2. Description of the Background
In image forming methods such as electrophotography, electrostatic recording and electrostatic printing, toner is generally used for developing an electrostatic latent image. The toner is required to be constituted of fine particles which are evenly charged and which have good fluidity. As to methods for developing an electrostatic latent image, two-methods are known, one of which is a developing method which employs a two-component developer, which includes a mixture of a toner and a carrier and the other of which is a developing method which uses a one-component developer which includes a toner but does not include a carrier. The developing method using a two-component developer has an advantage in that good images can be stably obtained, but the method has drawbacks which include that a carrier therein tends to deteriorate and the toner/carrier mixing ratio tends to change, resulting in deterioration of image qualities of the developed images.
In the image forming methods mentioned above, a developed toner image on a photoconductor is generally transferred to a transfer sheet and the transferred image is then fixed to obtain a fixed image. As for fixing methods, a heat roller fixing method is typically used in which a transfer sheet having a developed toner image thereon is heated while the sheet is pressed upon passage through a pair of heat rollers. This heat roller fixing method has an advantage in that a developed toner image can be quickly fixed, because the fixing method has high heat efficiency. However, a drawback of the method is the so-called offset problem which tends to occur in which an image which has transferred onto a transfer sheet, transfers instead to a heated roller and the image which has been transferred onto the roller is then transferred to another area of the transfer sheet or another transfer sheet, resulting in occurrence of undesirable images.
In attempting to solve these problems, Japanese Laid-Open Patent Publication No. 2-235067 discloses a developer which includes toner particles which include a release agent therein and thereon, each in a proper mixing ratio. The toner has releasability, however, the toner exhibits drawbacks such as poor fluidity and poor preservability, and has an uneven charge distribution which results in deterioration of image qualities, because the release agent is added onto the surface of the toner. In addition, Japanese Laid-Open Patent Publication No. 3-168649 discloses a developer including a toner in which a wax having low molecular weight, which serves as a release agent, is dispersed, by kneading with application of a large sheer strength is applied for a long time, in the binder resin of the toner so that the particle size of the dispersed wax is 1 .mu.m or less. This technique tends to avoid the offset problem by controlling the particle diameter of the dispersed release agent, but has drawbacks in that the resultant toner has high manufacturing costs, because the kneading apparatus, which is capable of applying such high shear strength is limited and expensive, and the productivity of the method is low because of the long kneading times required.
Further, demands currently exist for color reproduction methods which provide images having good color image qualities. For example, color toners such as yellow, magenta and cyan toners, and optionally a black toner, are used to prepare full color electrophotographic images. It is preferred that these color toners exhibit good light reflection properties without exhibiting random reflection, and good transparency so that any mixed color image prepared has a desired color when the color toners are overlaid. In addition, the color toners preferably have a relatively small particle size, so that developed color toner images have good resolution and sharpness. Therefore, it is necessary to improve the dispersion of a coloring agent in a binder resin of a color toner.
A toner is generally formed of a binder resin, a coloring agent (a dye, a pigment, a magnetizable material and/or the like), and a charge controlling agent. These toner materials are melted and mixed by kneading, and then pulverized after solidification by cooling. The pulverized toner is then classified to prepare toner particles having a desired particle diameter. The thus obtained toner particles are mixed with an additive such as a colloidal silica to prepare a toner having good fluidity. Extrusion type continuous kneaders having a screw, two-roller mills, three-roller mills, kneaders which can heat and press, or the like have been conventionally used for kneading these toner materials.
In a color toner, the dispersion of a coloring agent in a binder resin depends on the kneading process employed in manufacture of the color toner, and when the dispersion is not satisfactory, the resultant toner is poorly colored and has poor clearness and transparency, resulting in serious deterioration of the color reproducibility of formed color images. In the kneading apparatus mentioned above, the dispersion of a coloring agent in a toner is not satisfactory, and, therefore, images having good image qualities cannot be obtained. In attempting to solve this problem, so-called toner master batch methods are disclosed. For example, Japanese Laid-Open Patent Publication No. 3-155568 discloses a toner manufacturing method which includes a step in which a toner master batch is first prepared by melting and kneading a mixture of a portion of a binder resin, a coloring agent and a solvent and then cooling and pulverizing the mixture, and another step in which the toner master batch and the remainder of the binder resin are melted and kneaded. This method tends to improve the dispersion of the coloring agent in the binder resin by adding a solvent, but the method has a drawback in that it takes a long time to obtain a desired color toner in which the particles of the coloring agent are dispersed in the binder resin in a desired particle size. This is because the mixture, including the solvent, has relatively low melt viscosity and, therefore, it is difficult to improve the dispersion of the coloring agent. In addition, Japanese Laid-Open Patent Publication No. 8-146662 discloses a toner which includes a plurality of coloring agents which are different from each other only with respect to particle size distribution. This toner requires at least two toner master batches which are prepared by changing kneading conditions so that each toner master batch includes a coloring agent having a different particle size distribution. However, the thus obtained toner cannot necessarily produce good images and in addition the method has a drawback of low productivity, because it is necessary to prepare at least two master batches to make the toner. Therefore, these toners are not satisfactory. Because of these reasons, a need exists for a color toner which is useful for developing an electrostatic latent image which can produce full color images having good image qualities and which can be effectively manufactured.